This invention relates to flow measuring or sensing devices, and particularly to a non-intrusive thermal flow probe and flow measuring method.
A thermal flow probe determines the mass flow rate of a fluid flowing through a conduit by measuring heat transfer to the fluid. Prior thermal flow probes included a conduit and two resistive temperature sensors positioned within the conduit, a freestream sensor and a heated sensor positioned downstream of the freestream sensor. Both sensor elements intruded into the flow area of the conduit to maximize the heat transfer relationship with the fluid flowing therethrough. A heating device was connected to the heated sensor to elevate the temperature of the heated sensor element. With this arrangement, the heater input power required to maintain a constant temperature difference between the two sensors was proportional to the mass flow rate of a fluid flowing through the conduit and could be measured to provide a mass flow measurement.
Although intrusive thermal flow probes function well in many environments, their structural requirements prevent their use in certain applications. The most limiting structural requirement was that the sensor elements had to intrude into the flow area to provide a good heat transfer relationship with the fluid flowing through the conduit. Locating the sensors in the flow area subjected the sensors to damage from vibration produced by the flowing fluid. The intrusive sensor elements were also coated with particles from the fluid being measured which prevented the device from meeting "clean in place" standards for the food processing industry. Also, it was difficult to accurately measure the heater input power and this inaccuracy was reflected in the mass flow values obtained.
It is possible to simply remove the two sensors from the flow area and place them instead within the conduit wall or adjacent to the conduit. However, this arrangement raises other problems. First, with the sensor elements in contact with the conduit, heat from the heated sensor would be conducted to the upstream sensor and mask the true temperature reading. Also, sensors positioned outside the flow area could provide only boundary layer measurements which would cause the device to have an unfavorable time response as compared to the intrusive thermal flow probes.